1. Field of the Invention
The present invention relates to a method and an apparatus for cooling a steel strip which has been cooled through a cooling zone in a continuous heat treating line, in particular, for final cooling the strip by immersing in cooling water in a cooling tank.
2. Related Art Statement
There has been heretofore employed such method of cooling the steel strip by immersing in cooling water in a cooling tank for finally cooling the strip in the continuous heat treating line such as a continuous annealing line.
Such a cooling method is described, for example, in Japanese Patent Application Publication No. 11,931/57 wherein it is proposed that the temperature of cooling water is controlled to effect quick cooling without loss of aging characteristics and to be effectively recovered the heat energy of the steel strip by the cooling water. Further, in Japanese Patent Application Publication Nos. 11,932/57 and 11,933/57, there are disclosed cooling methods directed to saving and secondary utilization of the cooling water.
There has been however known that when the steel strip having a high temperature is cooled by immersing in cooling water in the cooling tank, the surface of the steel strip is often adhered with dirts from unknown cause.
Furthermore, it has been known that the tendency of adhering dirts to be surface of the steel strip becomes higher as in particular the temperature of the steel strip at the inlet of the cooling tank is higher and the amount of steel strip to be cooled in the cooling tank is greater.
Under the above circumstance, the conventional cooling process is therefore obliged to limit the amount of steel strip to be cooled or reduced the temperature of the steel strip at the inlet of the cooling tank in order to prevent dirts from adhering to the surface of the strip. There are however disadvantages that the limitation of the amount to be treated is resulted in reduction of productivity, on the other hand the lowering of the temperature of the steel strip at the inlet of the cooling tank and hence necessity of high cooling through the cooling zone usually arranged before the cooling tank is resulted in increment of cooling cost in the heat treating process.
An object of the present invention is to provide a method and an apparatus of finally cooling a steel strip capable of preventing dirts from adhering to the surface of the strip without the above mentioned disadvantages.
The inventor has investigated and found that
(i) the dirt adhesion is often produced when the temperature of strip (Ts) at the inlet of the cooling tank, the product of the line speed (v) and the thickness of strip (d), and/or the temperature of cooling water (Tw) are high; PA1 (ii) the compositions of the dirts are identical with the dirty suspensions in the cooling water in the cooling tank; and PA1 (iii) the dirts are adhered to only one side surface of the steel strip, which surface contacts with the surface of the sink-roll when the steel strip is wound around the sink-roll. PA1 Ts is the temperature of the steel strip at the inlet of the cooling tank (.degree.C.) PA1 Tw is the temperature of cooling water (.degree.C.) PA1 Cp is the specific heat of the steel strip (Kcal/kg.degree.C.) PA1 v is the feed speed of the steel strip (m/hr) PA1 d is the thickness of the steel strip (m) PA1 .alpha. is the coefficient of heat transfer (Kcal/m.sup.2 hr.degree.C.) PA1 .rho. is the density of the steel strip (kg/m.sup.3)
The inventor has further investigated and found that the surface of the steel strip is dirtied as a result in that in case of the steel strip still having a high temperature at the inlet of the cooling tank after cooling through the cooling zone in the heat treating line, the strip can not be sufficiently cooled with the cooling water in the cooling tank by the time of contacting with a first sink-roll so that a water film interposed between the surface of the sink-roll and the surface of the strip which is wound around the sink-roll is evaporated by the heat of the strip having a high temperature to deposit dirty suspensions included in the water on the surface of the strip.
The present invention bases on the above mentioned acknowledgement.
According to an aspect of the present invention, a method of cooling a steel strip which has been cooled through a cooling zone in a continuous heat treating line comprises step of cooling by immersing the strip in cooling water through around one or more sink-rolls in a cooling tank and the cooling of the steel strip immersed in the cooling water is controlled in accordance with the following formula: ##EQU2## here, l is the cooling length from the surface of the cooling water to the first one of the sink-rolls (m)
According to another aspect of the present invention, an apparatus for cooling a steel strip which has been cooled through a cooling zone in a continuous heat treating line comprises a cooling tank containing cooling water, one or more sink-rolls arranged in the cooling water to guide the steel strip in the cooling tank, a guide roll provided at the inlet of the cooling tank for guiding the steel strip from the outlet of the cooling zone to the first one of the sink-rolls in the cooling water, means for supplying cooling water to the cooling tank and a controller for controlling the cooling of the steel strip in accordance with the following formula: